1. Field of the Invention
This invention relates to a method and apparatus for reducing the NO.sub.x -content of flue gas in the burning of coal dust in coal-dust-fired combustion systems. The invention is particularly applicable to steam generating plants and plants wherein dry ash removal is practiced.
The excessive NO.sub.x -content of flue gas from the burning of fossil fuels is an acute environmental problem, and attempts are being made to reduce these emissions. In the Federal Republic of Germany, for example, legislation on large firing plants using coal-dust-fired steam generators with dry ash removal stipulates a maximum NO.sub.x -value of 650 mg/m.sup.3 for power station heat outputs of up to 300 MW and an NO.sub.x - value of 200 mg/m.sup.3 for outputs exceeding 300 MW.
Most old plants have NO.sub.x emission levels which exceed these values. This is in part attributable to the fact that in the interests of economy old plants are typically equipped with air preheaters for exchanging flue gas heat with combustion air to at least partially recycle the energy used in the combustion process. However, the burning of fossil fuels with air preheated to around 300.degree. C. results in the formation of very large quantities of NO.sub.x if conventional burners are used.
In principle, a reduction in the NO.sub.x content of the off-gas (effluent) stream entering the environment is obtainable by direct measures applied to the firing system, generally termed "primary measures", or by flue-gas cleaning plants, generally termed "secondary measures". The present invention is directed to a primary control measure, well adapted for economical retrofitting of existing coal-fired combustion systems.
2. Discussion of Related Art
In contrast to gas-fired systems, in which NO.sub.x is formed primarily by oxidation of the nitrogen in the air (thermal NO.sub.x), up to 60% of the NO.sub.x -emission of a coal-dust-fired system is derived from nitrogen in the fuel itself ("in-fuel" nitrogen).
In order to suppress the formation of NO.sub.x from in-fuel nitrogen, sub-stoichiometric quantities of combustion air have previously been employed in the burning of coal dust. One of the disadvantage of such a sub-stoichiometric approach is in the greatly increased formation of CO, which considerably increases the risk of CO corrosion damage to the boiler or fire box walls, and which is itself an environmental pollutant. It is also known that NO.sub.x is decomposed in the presence of gas constituents such as CO, H.sub.2, or hydrocarbons.
It is accordingly desirable to provide a method for reducing the NO.sub.x content of an off-gas stream of a coal-dust fired combustion system through primary means which occupy relatively little space, involve only a minimal loss of efficiency, and by which existing plants can be inexpensively retrofitted.